I heard that Newton used the Moon acceleration to come up with his universal law of gravity, but in order to calculate its acceleration you need the distance to the Moon. Whose measurement of this distance was used and how did they measure it?


1 Answer 1


In Newton's time distance to the moon was not a problem: since the ancient times this distance (in terms of the Earth radius) was measured using parallax. Newton used the value 60 for the ratio of Moon's orbit to the Earth radius. This is close to the value 59 which can be found in Ptolemy.

Newton's problem was with Earth radius which was not known to young Newton with sufficient accuracy. For this reason his first check of the gravitation law gave incorrect result, and Newton abandoned the subject. Only when a more precise measurement became available (by Picard), Newton saw that his gravitation theory is correct.

Reference. The article Newton's moon test by Douglas W. MacDougal in the book Newton's gravity, Springer 2012, gives the following values of the distance available in Newton's time: 59 (Ptolemy), 60 (Huygens), 60 1/3 (Copernicus), 60 2/5 (Street) and 60 1/2 (Tyho Brahe). Newton does not mention all these authors, just taking 60 as a "well-known" number.

Remarks. Since the question of size the the Earth is raised in the comments, let me comment on this too. The best measurement which Newton could use is by Snell (6369 km), but Newton did not know this result at the time of calculation.

Wikipedia claims that Biruni's measurement was accurate to 17 km (0.2%), which is hard to believe, given his method described in Wikipedia. I followed the links from Wikipedia article, and could not trace the reference for this unbelievably precise number. Quick search hows various figures attributed to al-Biruni, and the ridiculously precise value cited in Wikipedia is certainly made up.

  • $\begingroup$ my understanding was that al Al Biruni had already calculated the radius of the earth to a reasonable degree of accuracy before newton's time, did the western world not have access to this information? $\endgroup$
    – cal
    Jan 6, 2018 at 14:36
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    $\begingroup$ None of Biruni's books were translated into any European language until the second half of the 19th century. There were no mediaeval Latin or Hebrew translations. @cal. $\endgroup$
    – fdb
    Jan 6, 2018 at 18:15
  • $\begingroup$ I doubt that Newton had access to Al Biruni's work. In his first calculation he used a very imprecise number, probably even worse than the best "available in the West" . Up-to-date information was difficult to obtain at that time, even when it was somehow "available" in theory. $\endgroup$ Jan 6, 2018 at 21:29
  • $\begingroup$ Can you give some references which describe what methods of measurement Copernicus, Street and Brahe used? $\endgroup$
    – student
    Nov 10, 2020 at 14:38
  • $\begingroup$ @Student: the distance to the Moon was measured since Ptolemy by measuring Moons parallax. For this a "parallactic instrument" described by Ptolemy was used. There was no substantial improvement in this instrument, except that Brahe was able to achieve a better precision by good workmanship. $\endgroup$ Nov 10, 2020 at 15:10

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